A. Field of the Invention
The invention relates to a radio broadcasting system with code signalling having a multiplex signal which is frequency modulated on a main carrier wave being transmitted at the transmission side, wherein the multiplex signal comprises: an audio frequency information signal; a stereo information signal modulated on a suppressed stereo subcarrier wave, a stereo pilot, whose frequency is situated between the frequency spectra of the audio frequency information signal and of the modulated stereo information signal and which serves for demodulating the stereo information signal, as well as a code signal which is binary phase-modulated on a further subcarrier wave which is a harmonic of a subharmonic of the stereo pilot, and which is derived from the same frequency source as the stereo pilot at the transmission side.
B. Description of the Prior Art
Such a radio broadcasting system with code signalling, which may for example be used for transmitter identification or for paging purposes or for the transmission of other kinds of information, has been described in Applicant's previous U.S. patent application Ser. No. 878,666, to which explicit reference is made. The system described in said Application exploys a subcarrier wave which is phase-modulated in binary manner with the transmitter identification signal (so-called P.S.K. signal), which subcarrier wave is situated above the frequency spectrum (23-53 KHz) of the modulated stereo information signal, for example with a subcarrier frequency 7/2 or of 16/5 times the stereo-pilot frequency (66.5 KHz or 60.8 KHz, respectively).
However, it has been found that, in the case of a transmission in accordance with said previously proposed system, disturbances in existing receivers are almost inevitable. The non-linear phase characteristic of the intermediate-frequency section of the receiver gives rise to interference products of the modulated transmitter identification signal with the other components of the multiplex signal, in particular the stereo pilot, and these interference products may become audible directly, or in the case of stereo reception via the stereo demodulation.
Furthermore, it has been found that in a number of stereo receivers, stereo detection is effected with not-sufficiently symmetrical 38 KHz signal. As a result of this the detection signal also contains a 76-KHz component, which in conjunction with the transmitter identification signal of for example 66.5 KHz, gives rise to an audible whistle tone.